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Abstract:

The present invention provides a tire rubber composition with which the
grip performance, the abrasion resistance, and the performance in terms
of rolling resistance are improved in a balanced manner. The present
invention relates to a tire rubber composition comprising: a modified
diene rubber A which is modified with a specific acrylamide compound; and
a modified diene rubber B which is modified with a specific modifying
compound alone or together with a specific silicon or tin compound, a
weight average molecular weight of a total of the modified diene rubbers
A and B being 300,000 to 1,400,000.

Claims:

1. A tire rubber composition comprising: a modified diene rubber A which
is modified with an acrylamide compound represented by the following
formula (I); and a modified diene rubber B which is modified with a
modifying compound represented by the following formula (III) alone or
together with a silicon or tin compound represented by the following
formula (II), a weight average molecular weight of a total of the
modified diene rubbers A and B being 300,000 to 1,400,000, the formula
(I) being as follows: ##STR00005## wherein R1 represents hydrogen
or a methyl group, R2 and R3 each represent an alkyl group, and
n represents an integer; the formula (II) being as follows:
RaMXb (II) wherein R represents an alkyl, alkenyl,
cycloalkenyl, or aromatic hydrocarbon group, M represents silicon or tin,
X represents halogen, a represents an integer of 0 to 2, and b represents
an integer of 2 to 4; and the formula (III) being as follows:
##STR00006## wherein R4 to R6 are the same as or different
from each other and each represent a C1-8 alkyl group, R7 to
R12 are the same as or different from each other and each represent
a C1-8 alkoxy or alkyl group, and p to r are the same as or
different from each other and each represent an integer of 1 to 8.

2. The tire rubber composition according to claim 1, wherein the modified
diene rubbers A and B are a mixture which is prepared by reacting an
active conjugated diene polymer having an alkali metal end with the
acrylamide compound and with the modifying compound alone or together
with the silicon or tin compound, the active conjugated diene polymer
being obtained by polymerizing a conjugated diene monomer alone or
together with an aromatic vinyl monomer, in the presence of an alkali
metal catalyst in a hydrocarbon solvent.

3. The tire rubber composition according to claim 1, wherein, in the
modifying compound, R4 to R6 each are a methyl, ethyl, propyl
or butyl group, R7 to R12 each are a methoxy, ethoxy, propoxy
or butoxy group, and p to r each are an integer of 2 to 5.

4. The tire rubber composition according to claim 1, further comprising
an aromatic vinyl polymer obtained by polymerizing at least one of
α-methylstyrene and styrene.

5. A tire rubber composition comprising a mixture prepared by reacting an
active conjugated diene polymer having an alkali metal end with two or
more modifying agents, the active conjugated diene polymer obtained by
polymerizing a conjugated diene monomer alone or together with an
aromatic vinyl monomer, in the presence of an alkali metal catalyst in a
hydrocarbon solvent.

6. A pneumatic tire comprising a tread produced using the tire rubber
composition according to claim 1.

7. A pneumatic tire comprising a tread produced using the tire rubber
composition according to claim 5.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a tire rubber composition and a
pneumatic tire produced using the rubber composition.

BACKGROUND ART

[0002] Recently, pneumatic tires for automobiles have been required to
have a wide range of performance properties, such as fuel economy (the
performance in terms of rolling resistance), grip performance (handling
stability) and abrasion resistance. Various methods have been proposed to
achieve improvement in these performance properties.

[0003] For example, a known method for improving the abrasion resistance
is to use natural rubber or butadiene rubber as a rubber component. This
method, however, tends to deteriorate the grip performance. An exemplary
method for enhancing both the abrasion resistance and grip performance is
to increase the amount of a reinforcing agent such as carbon black and
silica. In this method, however, the performance in terms of rolling
resistance tends to be deteriorated.

[0004] Other methods for improving the abrasion resistance and grip
performance include a method of adding a polymer obtained by
polymerization of only an aromatic vinyl monomer, and a method of adding
a hydrogenation product of a low-molecular weight aromatic
vinyl-conjugated diene copolymer (Patent Documents 1 and 2). However,
there still remains a demand to improve the grip performance and abrasion
resistance as well as the performance in terms of rolling resistance in a
balanced manner because these properties usually conflict with each other
and it is difficult to improve these simultaneously. [0005] Patent
Document 1: JP-A 2007-112994 [0006] Patent Document 2: JP-A 2005-225946

SUMMARY OF THE INVENTION

[0007] The present invention aims to provide a tire rubber composition
with which the above problem is solved and the grip performance, the
abrasion resistance, and the performance in terms of rolling resistance
are improved in a balanced manner.

[0008] The present invention relates to a tire rubber composition
comprising: a modified diene rubber A which is modified with an
acrylamide compound represented by the following formula (I); and a
modified diene rubber B which is modified with a modifying compound
represented by the following formula (III) alone or together with a
silicon or tin compound represented by the following formula (II), a
weight average molecular weight of a total of the modified diene rubbers
A and B being 300,000 to 1,400,000, the formula (I) being as follows:

##STR00001##

[0009] wherein R1 represents hydrogen or a methyl group, R2 and
R3 each represent an alkyl group, and n represents an integer;

the formula (II) being as follows:

RaMXb (II)

[0010] wherein R represents an alkyl, alkenyl, cycloalkenyl, or aromatic
hydrocarbon group, M represents silicon or tin, X represents halogen, a
represents an integer of 0 to 2, and b represents an integer of 2 to 4;
and the formula (III) being as follows:

##STR00002##

[0011] wherein R4 to R6 are the same as or different from each
other and each represent a C1-8 alkyl group, R7 to R12 are
the same as or different from each other and each represent a C1-8
alkoxy or alkyl group, and p to r are the same as or different from each
other and each represent an integer of 1 to 8.

[0012] The modified diene rubbers A and B are preferably a mixture which
is prepared by reacting an active conjugated diene polymer having an
alkali metal end with the acrylamide compound and with the modifying
compound alone or together with the silicon or tin compound, the active
conjugated diene polymer being obtained by polymerizing a conjugated
diene monomer alone or together with an aromatic vinyl monomer, in the
presence of an alkali metal catalyst in a hydrocarbon solvent.

[0013] Preferably, in the modifying compound represented by the formula
(III), R4 to R6 each are a methyl, ethyl, propyl or butyl
group, R7 to R12 each are a methoxy, ethoxy, propoxy or butoxy
group, and p to r each are an integer of 2 to 5.

[0014] The rubber composition preferably further comprises an aromatic
vinyl polymer obtained by polymerizing at least one of
α-methylstyrene and styrene.

[0015] The present invention also relates to a tire rubber composition
comprising a mixture prepared by reacting an active conjugated diene
polymer having an alkali metal end with two or more modifying agents, the
active conjugated diene polymer being obtained by polymerizing a
conjugated diene monomer alone or together with an aromatic vinyl
monomer, in the presence of an alkali metal catalyst in a hydrocarbon
solvent.

[0016] The present invention further relates to a pneumatic tire
comprising a tread produced using the tire rubber composition.

[0017] The tire rubber composition according to the present invention
contains a modified diene rubber A having an end modified with a specific
acrylamide compound, and a modified diene rubber B which is modified with
a specific modifying compound alone or together with a silicon or tin
compound. Also in the tire rubber composition, the weight average
molecular weight of a total of the rubbers A and B is in a specific
range. Such a tire rubber composition enables to synergistically improve
the performance balance in terms of grip performance, abrasion
resistance, and rolling resistance.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] The tire rubber composition of the present invention contains
modified diene rubbers A and B, and the weight average molecular weight
of a total of the rubbers A and B is in a specific range.

[0019] The modified diene rubber A is a diene rubber modified with an
acrylamide compound represented by the formula (I):

##STR00003##

[0020] wherein R1 represents hydrogen or a methyl group, R2 and
R3 each represent an alkyl group, and n represents an integer. This
rubber constitutes a diene rubber having a polymer end modified with the
acrylamide compound.

[0021] In the formula (I), R2 and R3 each are preferably a
C1-4 alkyl group, and n is preferably an integer of 2 to 5.

[0023] The modified diene rubber B is a diene rubber modified with a
silicon or tin compound represented by the formula (II) and a modifying
compound represented by the formula (III), or a diene rubber modified
with a modifying compound represented by the formula (III). The former
rubber is a diene rubber in which a polymer end is coupled with the
silicon or tin compound and then modified with the modifying compound.
The latter rubber is a diene rubber having a polymer end modified with
the modifying compound.

RaMXb (II)

[0024] In the formula (II), R represents an alkyl, alkenyl, cycloalkenyl,
or aromatic hydrocarbon group, M represents silicon or tin, X represents
halogen, a represents an integer of 0 to 2, and b represents an integer
of 2 to 4.

##STR00004##

[0025] In the formula (III), R4 to R6 are the same as or
different from each other and each represent a C1-8 alkyl group,
R7 to R12 are the same as or different from each other and each
represent a C1-8 alkoxy or alkyl group, and p to r are the same as
or different from each other and each represent an integer of 1 to 8.

[0026] The silicon or tin compound represented by the formula (II)
functions as a coupling agent for diene rubber.

[0028] In the formula (III), R4 to R6 each are preferably a
methyl, ethyl, propyl or butyl group, R7 to R12 each are
preferably a methoxy, ethoxy, propoxy or butoxy group, and p to r each
are preferably an integer of 2 to 5. Such a structure improves
performance in terms of grip performance, abrasion resistance, and
rolling resistance in a balanced manner.

[0029] Specific examples of the modifying compound represented by the
formula (III) include 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate,
1,3,5-tris(3-triethoxysilylpropyl)isocyanurate,
1,3,5-tris(3-tripropoxysilylpropyl)isocyanurate, and
1,3,5-tris(3-tributoxysilylpropyl)isocyanurate. In particular,
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate is preferred because it
improves performance in terms of grip performance, abrasion resistance,
and rolling resistance in a balanced manner.

[0030] The modified diene rubbers A and B may, for example, be prepared as
a mixture by producing the rubbers A and B separately and blending them.
In such a case, the modified diene rubbers A and B each may be produced
as described below.

[0031] The modified diene rubber A may be produced by reacting an active
conjugated diene polymer having an alkali metal end with an acrylamide
compound represented by the formula (I). Here, the active conjugated
diene polymer having an alkali metal end is produced by polymerizing a
conjugated diene monomer alone or together with an aromatic vinyl
monomer, in the presence of an alkali metal catalyst in a hydrocarbon
solvent.

[0032] Examples of the conjugated diene monomer include 1,3-butadiene,
isoprene, 1,3-pentadiene (piperylene), 2,3-dimethyl-1,3-butadiene, and
1,3-hexadiene. Considering the physical properties of the resulting
polymer and the availability for industrial purposes, 1,3-butadiene and
isoprene are preferred among these.

[0033] Examples of the aromatic vinyl monomer include styrene,
α-methylstyrene, vinyltoluene, vinylnaphthalene, divinylbenzene,
trivinylbenzene, and divinylnaphthalene. Considering the physical
properties of the resulting polymer and the availability for industrial
purposes, styrene is preferred among these.

[0034] The hydrocarbon solvent is not particularly limited as long as it
is a solvent that does not deactivate an alkali metal catalyst. Examples
thereof include aliphatic hydrocarbons, aromatic hydrocarbons, and
alicyclic hydrocarbons. Specific examples thereof include those with 3 to
12 carbon atoms such as propane, n-butane, iso-butane, n-pentane,
iso-pentane, n-hexane, cyclohexane, benzene, toluene, and xylene.

[0036] The monomer to be polymerized may be the conjugated diene monomer
either alone or together with the aromatic vinyl monomer. When the
conjugated diene monomer and the aromatic vinyl monomer are used in
combination, the ratio of these monomers (conjugated diene
monomer/aromatic vinyl monomer) is preferably 50/50 to 90/10, and more
preferably 55/45 to 85/15, on a mass basis.

[0037] In the polymerization, materials usually used, such as an alkali
metal catalyst, a hydrocarbon solvent, a randomizer, an agent for
controlling the vinyl bond content of the conjugated diene units, and the
like may be used, and the method for producing the polymer is not
particularly limited.

[0038] Various Lewis basic compounds may be used for controlling the vinyl
bond content of the conjugated diene units. Considering the availability
for industrial purposes, ether compounds and tertiary amines are
preferred. Examples of the ether compounds include cyclic ethers such as
tetrahydrofuran, tetrahydropyran, and 1,4-dioxane; aliphatic monoethers
such as diethyl ether and dibutyl ether; and aliphatic diethers such as
ethylene glycol dimethyl ether. Examples of the tertiary amines include
triethylamine and tripropylamine.

[0039] On the occasion of adding the acrylamide compound to the active
conjugated diene polymer having an alkali metal end to produce the
modified diene rubber A, the amount of the acrylamide compound is usually
0.05 to 10 mol, and preferably 0.2 to 2 mol per mol of the alkali metal
catalyst used for the addition of the alkali metal.

[0040] Since the reaction between the acrylamide compound and the active
conjugated diene polymer having an alkali metal end rapidly proceeds, the
reaction temperature and the reaction time can be selected from wide
ranges. Typically, the reaction temperature is from room temperature to
100° C. and the reaction time is from few seconds to several
hours. Any method may be employed for the reaction as long as the active
conjugated diene polymer and the acrylamide compound are brought into
contact. Mention may be made of, for example, a method in which the diene
polymer is produced using an alkali metal catalyst, and a predetermined
amount of the acrylamide compound is then added to the polymer solution.

[0041] After completion of the reaction, the modified diene polymer can be
coagulated by a usual coagulation technique used in the production of
rubber by solution polymerization, such as addition of a coagulant or
steam coagulation, and then separated from the reaction solvent. Further,
the coagulation temperature is not at all limited. The resulting modified
diene rubber A has a molecular end to which the acrylamide compound has
been introduced.

[0042] Meanwhile, the modified diene rubber B may be produced by: (a)
reacting an active conjugated diene polymer having an alkali metal end
with a silicon or tin compound (coupling agent) represented by the
formula (II) and then with a modifying compound represented by the
formula (III); or (b) reacting the active conjugated diene polymer having
an alkali metal end with the modifying compound represented by the
formula (III). Here, the active conjugated diene polymer having an alkali
metal end is obtained by polymerizing a conjugated diene monomer alone or
together with an aromatic vinyl monomer, in the presence of an alkali
metal catalyst in a hydrocarbon solvent.

[0043] The active conjugated diene polymer having an alkali metal end may
be obtained in the same manner as in the production of the modified diene
rubber A. In the process (a), the silicon or tin compound is used usually
in the range of 0.01 to 0.4 equivalents of the halogen atoms per
equivalent of the terminal alkali metal atom of the active conjugated
diene polymer. The coupling reaction is usually carried out at a
temperature range of 20° C. to 100° C. The reaction of the
modifying compound in the process (a) or (b) may be carried out in the
same manner as in the reaction of the acrylamide compound mentioned
above. The modified diene rubber B obtained has a molecular end to which
the modifying compound has been introduced.

[0044] The modified diene rubbers A and B are preferably a mixture which
is prepared through single-batch production of the rubbers A and B. In
such a case, for example, the mixture may be prepared by reacting the
active conjugated diene polymer having an alkali metal end with the
acrylamide compound and with the modifying compound alone or together
with the silicon or tin compound.

[0045] More specifically, the mixture may be prepared by, for example, the
following process (c) or (d). In the process (c), an active conjugated
diene polymer having an alkali metal end is produced by the same method
as described above. Next, an acrylamide compound is added to the polymer
solution. Then, a silicon or tin compound (coupling agent) is optionally
added and a modifying compound is subsequently added thereto.
Alternatively, in the process (d), after the production of the active
conjugated diene polymer having an alkali metal end, an acrylamide
compound, a modifying compound, and optionally a silicon or tin compound
are simultaneously added to the polymer solution.

[0046] In such cases, the reactions with an acrylamide compound and with a
modifying compound and the coupling reaction may be carried out in the
same manner as mentioned above. The resulting mixture contains the
modified diene rubber A having a molecular end to which the acrylamide
compound has been introduced, and the modified diene rubber B having a
molecular end to which the modifying compound has been introduced.

[0047] The weight average molecular weight (Mw) of a total of the modified
diene rubbers A and B used in the rubber composition of the present
invention (the weight average molecular weight measured for the entire
composition consisting of the modified diene rubbers A and B) is not less
than 300,000, preferably not less than 500,000, and more preferably not
less than 600,000. The Mw is not more than 1,400,000, preferably not more
than 1,200,000, and more preferably not more than 1,000,000. The Mw in
that range improves performance in terms of grip performance, abrasion
resistance, and rolling resistance in a balanced manner.

[0048] The molecular weight distribution (Mw/Mn) of a total of the
modified diene rubbers A and B is preferably not more than 4, more
preferably not more than 3.5, and still more preferably not more than 3.
If the Mw/Mn is more than 4, the dispersibility of filler tends to be
lowered to increase tan δ (deteriorate the performance in terms of
rolling resistance).

[0049] As used herein, the number average molecular weight (Mn) and the
weight average molecular weight (Mw) of the rubbers and the aromatic
vinyl polymer mentioned later are determined relative to polystyrene
standards based on measurement values obtained by a gel permeation
chromatograph (GPC) (GPC-8000 series produced by TOSOH CORPORATION,
detector: differential refractometer, column: TSKGEL SUPERMULTIPORE HZ-M
produced by TOSOH CORPORATION).

[0050] The modified diene rubbers A and B each are preferably a modified
butadiene rubber (modified BR) or a modified styrene butadiene rubber
(modified SBR), and more preferably a modified SBR, because they improve
performance in terms of grip performance, abrasion resistance, and
rolling resistance in a balanced manner.

[0051] In the case where the modified diene rubbers A and B are modified
SBRs, the vinyl bond content of the butadiene units of a total of the
rubbers A and B is preferably not less than 20% by mass, and more
preferably not less than 25% by mass. A modified diene rubber having a
vinyl bond content of less than 20% by mass tends to be difficult to
polymerize (produce). The vinyl bond content is preferably not more than
60% by mass, and more preferably not more than 55% by mass. If the vinyl
bond content is more than 60% by mass, the dispersibility of filler tends
to be lowered. The vinyl bond content (1,2-butadiene unit content) herein
can be determined by infrared absorption spectrometry.

[0052] In the case where the modified diene rubbers A and B are modified
SBRs, the styrene content of a total of the rubbers A and B is preferably
not less than 15% by mass, and more preferably not less than 25% by mass.
If the styrene content is less than 15% by mass, the grip performance
tends to be deteriorated. The styrene content is preferably not more than
50% by mass, and more preferably not more than 45% by mass. If the
styrene content is more than 50% by mass, the abrasion resistance tends
to be deteriorated. The styrene content herein is determined by 1H
NMR.

[0053] In the rubber composition of the present invention, the compounding
ratio of the modified diene rubbers A and B (weight ratio of A/B) is
preferably 5/95 to 95/5, more preferably 10/90 to 90/10, and still more
preferably 20/80 to 80/20. The ratio below the lower limit tends to
deteriorate the performance in terms of rolling resistance, while the
ratio above the upper limit tends to deteriorate the abrasion resistance.
In both cases, the balance among the above-mentioned performance
properties tends to be lowered.

[0054] The total amount of the modified diene rubbers A and B in 100% by
mass of the rubber component of the rubber composition is preferably not
less than 2% by mass, more preferably not less than 5% by mass, and still
more preferably not less than 10% by mass. If the total amount is less
than 2% by mass, the performance may not be sufficiently improved in
terms of rolling resistance and abrasion resistance. The upper limit of
the total amount is not particularly limited, and may be 100% by mass.
The upper limit is preferably not more than 90% by mass, and more
preferably not more than 80% by mass.

[0055] Examples of other rubbers which may be contained in the rubber
component in the present invention include diene rubbers such as natural
rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene
butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), chloroprene
rubber (CR), butyl rubber (IIR), halogenated butyl rubber (X-IIR), and
styrene-isoprene-butadiene copolymer rubber (SIBR). In particular, SBR
and BR are preferably used because they are highly compatible and highly
contribute to better balance among the performance properties.

[0056] As the SBR, those commonly used in the tire industry, such as
emulsion polymerized styrene butadiene rubber (E-SBR) and solution
polymerized styrene butadiene rubber (S-SBR), may be used. Examples of
the BR include BR having high cis content and BR containing syndiotactic
polybutadiene crystals.

[0057] In the case where SBR (unmodified) is added, the SBR content is
preferably not less than 20% by mass, and more preferably not less than
50% by mass. The content less than 20% by mass tends to deteriorate the
grip performance. The SBR content is preferably not more than 90% by
mass, and more preferably not more than 70% by mass. The content more
than 90% by mass tends to deteriorate the abrasion resistance.

[0058] In the case where BR (unmodified) is added, the BR content is
preferably not less than 5% by mass, and more preferably not less than
10% by mass. The content less than 5% by mass tends to deteriorate the
abrasion resistance. The BR content is preferably not more than 40% by
mass, and more preferably not more than 30% by mass. The content more
than 40% by mass tends to deteriorate the grip performance.

[0059] The rubber composition of the present invention preferably contains
at least one of carbon black and silica (preferably both ingredients) as
filler. In the present invention, the use of the modified diene rubbers A
and B significantly improves the dispersibility of filler such as silica
and carbon black. Therefore, the performance balance in terms of grip
performance, abrasion resistance, and rolling resistance is
synergistically improved.

[0060] Such improvement is presumably achieved by the following actions.

[0061] The rubber A having an end modified with the acrylamide compound
has higher interaction with silica or carbon black. If the rubber A is
only used, however, filler agglomerates cannot be expected to break
because the ratio of low molecular weight ingredients is large. As a
result, it is difficult to enhance the dispersibility of filler. In
contrast, in the present invention, since the rubber B having an end
modified with the modifying compound is further used, the interaction
with filler, especially with silica, is further increased. In addition,
modified end groups of molecules of the rubber B interact with each
other, while maintaining the interaction with silica, so that polymers
are coupled to have a higher molecular weight. Then, filler agglomerates
are sufficiently broken, and therefore the rubbers A and B efficiently
and synergistically exert the effect of enhancing the dispersibility of
filler. Presumably for this reason, the balance among the performance
properties is significantly improved.

[0062] Use of carbon black increases reinforcement to improve the abrasion
resistance and grip performance. The carbon black is not particularly
limited, and examples thereof include GPF, FEF, HAF, ISAF, and SAF. Each
carbon black may be used alone, or two or more thereof may be used in
combination.

[0063] The nitrogen adsorption specific surface area (N2SA) of carbon
black is preferably not less than 50 m2/g, and more preferably not
less than 100 m2/g. The N2SA less than 50 m2/g tends not
to provide sufficient reinforcement. The N2SA of carbon black is
preferably not more than 200 m2/g, and more preferably not more than
150 m2/g. The N2SA more than 200 m2/g tends to deteriorate
the performance in terms of rolling resistance because such carbon black
is less likely to be dispersed. Here, the nitrogen adsorption specific
surface area of carbon black is determined in accordance with the A
method of JIS K6217.

[0064] The dibutyl phthalate (DBP) oil absorption of carbon black is
preferably not less than 60 ml/100 g, and more preferably not less than
100 ml/100 g. The DBP oil absorption less than 60 ml/100 g tends not to
provide sufficient reinforcement. The DBP oil absorption of carbon black
is preferably not more than 150 ml/100 g, and more preferably not more
than 120 ml/100 g. The DBP oil absorption more than 150 ml/100 g tends to
lower the processability and dispersibility.

[0065] Here, the DBP oil absorption of carbon black is determined by the
measuring method of JIS K6217-4.

[0066] The carbon black content is preferably not less than 10 parts by
mass, more preferably not less than 20 parts by mass, and sill more
preferably not less than 30 parts by mass, for each 100 parts by mass of
the rubber component. The carbon black content less than 10 parts by mass
tends not to provide sufficient reinforcement. The carbon black content
is preferably not more than 100 parts by mass, more preferably not more
than 80 parts by mass, and still more preferably not more than 60 parts
by mass, for each 100 parts by mass of the rubber component. The carbon
black content more than 100 parts by mass tends to deteriorate the
performance in terms of rolling resistance because such an amount of
carbon black is less likely to be dispersed.

[0067] Use of silica increases reinforcement and, at the same time,
improves the performance in terms of rolling resistance. Examples of the
silica include silica produced by a wet method and silica produced by a
dry method. Each silica may be used alone, or two or more thereof may be
used in combination.

[0068] The nitrogen adsorption specific surface area (N2SA) of silica
is preferably not less than 120 m2/g, and more preferably not less
than 150 m2/g. The N2SA less than 120 m2/g tends not to
provide sufficient reinforcement. The N2SA of silica is preferably
not more than 250 m2/g, and more preferably not more than 200
m2/g. The N2SA more than 250 m2/g tends to deteriorate the
performance in terms of rolling resistance because such silica has lower
dispersibility so that the hysteresis loss increases. Here, the nitrogen
adsorption specific surface area of silica is determined in accordance
with the BET method of ASTM D3037-81.

[0069] The silica content is preferably not less than 15 parts by mass,
more preferably not less than 25 parts by mass, and still more preferably
not less than 35 parts by mass, for each 100 parts by mass of the rubber
component. The silica content less than 15 parts by mass tends not to
provide sufficient reinforcement. The silica content is preferably not
more than 100 parts by mass, more preferably not more than 85 parts by
mass, and still more preferably not more than 65 parts by mass, for each
100 parts by mass of the rubber component. The silica content more than
100 parts by mass tends to deteriorate the performance in terms of
rolling resistance because such an amount of silica has lower
dispersibility.

[0070] In the rubber composition of the present invention, the total
amount of carbon black and silica is preferably not less than 40 parts by
mass, and more preferably not less than 70 parts by mass, for each 100
parts by mass of the rubber component. The total amount less than 40
parts by mass tends not to provide sufficient reinforcement. The total
amount is preferably not more than 150 parts by mass, and more preferably
not more than 110 parts by mass, for each 100 parts by mass of the rubber
component. The total amount more than 150 parts by mass tends to lower
the dispersibility of filler.

[0071] In the present invention, a silane coupling agent may be used in
combination with silica. Examples of the silane coupling agent include
bis(3-triethoxysilylpropyl)tetrasulfide,
bis(3-triethoxysilylpropyl)trisulfide, and
bis(3-triethoxysilylpropyl)disulfide. In particular,
bis(3-triethoxysilylpropyl)tetrasulfide is preferred because the
reinforcing effect is more enhanced.

[0072] The silane coupling agent content is preferably not less than 1
part by mass, and more preferably not less than 2 parts by mass, for each
100 parts by mass of silica. The content less than 1 part by mass tends
to increase the viscosity of the unvulcanized rubber composition to lower
the processability. The silane coupling agent content is preferably not
more than 20 parts by mass, and more preferably not more than 15 parts by
mass, for each 100 parts by mass of silica. The content more than 20
parts by mass tends not to provide an effect consistent with the cost
increase.

[0073] The rubber composition of the present invention preferably contains
a specific aromatic vinyl polymer, namely, a resin obtained by
polymerizing at least one of α-methylstyrene and styrene. This
improves the grip performance while maintaining performance in terms of
rolling resistance and abrasion resistance, so that the balance among the
performance properties is further improved. It should be noted that, in
the present invention, the aromatic vinyl polymer is not included in the
rubber component.

[0074] To produce the aromatic vinyl polymer, at least one of styrene and
α-methylstyrene is used as an aromatic vinyl monomer (unit). The
polymer may be either a homopolymer of each monomer or a copolymer of
both monomers. In particular, having better cost efficiency and higher
processability, and being excellent in the balance among the performance
properties, a homopolymer of α-methylstyrene or a copolymer of
α-methylstyrene and styrene is preferred, and a copolymer of
α-methylstyrene and styrene is particularly preferred.

[0075] The softening point of the aromatic vinyl polymer is preferably not
higher than 100° C., more preferably not higher than 92°
C., and still more preferably not higher than 88° C. The softening
point higher than 100° C. tends to deteriorate the abrasion
resistance and grip performance. The softening point is preferably not
lower than 30° C., more preferably not lower than 60° C.,
and still more preferably not lower than 75° C. The softening
point lower than 30° C. tends to deteriorate the grip performance.
Here, the softening point herein is measured as set forth in JIS K6220 by
using a ring and ball softening point measuring apparatus, and refers to
the temperature at which the ball falls.

[0076] The weight average molecular weight (Mw) of the aromatic vinyl
polymer is preferably not less than 500, and more preferably not less
than 800. The Mw less than 500 is less likely to achieve sufficient
improvement in terms of rolling resistance and grip performance. The Mw
of the aromatic vinyl polymer is preferably not more than 3,000, and more
preferably not more than 2,000. The Mw more than 3,000 tends to
deteriorate the performance in terms of rolling resistance.

[0077] The aromatic vinyl polymer content is preferably not less than 1
part by mass, and more preferably not less than 2 parts by mass, for each
100 parts by mass of the rubber component. The content less than 1 part
by mass may fail to achieve improvement in grip performance. The aromatic
vinyl polymer content is preferably not more than 100 parts by mass, and
more preferably not more than 20 parts by mass, for each 100 parts by
mass of the rubber component. The content more than 100 parts by mass
tends to deteriorate performance in terms of rolling resistance and
abrasion resistance.

[0078] The rubber composition of the present invention may appropriately
contain additives according to need, such as oils, antioxidants,
vulcanizing agents, vulcanization accelerators, and vulcanization
accelerator aids, in addition to the above ingredients.

[0079] In another aspect, the present invention may provide a tire rubber
composition containing a mixture prepared by reacting an active
conjugated diene polymer having an alkali metal end with two or more
modifying agents. Here, the active conjugated diene polymer having an
alkali metal end is obtained by polymerizing a conjugated diene monomer
alone or together with an aromatic vinyl monomer, in the presence of an
alkali metal catalyst in a hydrocarbon solvent.

[0080] That is, though the earlier description illustrates a mixture
prepared by reacting the active conjugated diene polymer with specific
modifying agents, the present invention is not limited to such an
embodiment and includes a mixture prepared by a reaction with any two or
more modifying agents. The mixture may, for example, be prepared by a
single-batch reaction of an active conjugated diene polymer having an
alkali metal end, which is produced in the same manner as mentioned
above, with two or more conventionally known terminal modifying agents.
The use of such a mixture prepared in a single batch achieves improvement
in the balance among the performance properties.

[0081] The rubber composition of the present invention can be produced by
a usual method. Specifically, for example, the above components are
kneaded using a kneading apparatus such as a Banbury mixer, a kneader,
and an open roll mill, and the kneaded mixture is then vulcanized,
whereby the rubber composition can be produced. The rubber composition is
suitably used for tire treads, and tires produced using the rubber
composition are suitably used for automobiles, commercial vehicles,
two-wheel vehicles, and the like.

[0082] The pneumatic tire of the present invention can be produced by a
usual method with use of the rubber composition. Specifically, before
vulcanization, the rubber composition containing the above components is
extruded and processed into the shape of a tread or the like, and then
assembled with other tire components in a usual manner in a tire building
machine to form an unvulcanized tire. Then, the unvulcanized tire is
heated and pressed in a vulcanizer, whereby the pneumatic tire of the
present invention can be produced.

EXAMPLES

[0083] The present invention will be more specifically described based on
examples, but the present invention is not limited to these examples.

[0084] In the following, the chemical agents used in examples and
comparative examples are listed.

[0085] Modified diene rubbers A and B: see the following Preparation
Examples 1 to 10 (oil content of each rubber: 15% by mass)

[0086] A 20-L stainless steel polymerization reactor was cleaned and
dried, and the air therein was replaced with dry nitrogen. To the reactor
were then added 1,3-butadiene (548 g), styrene (235 g), tetrahydrofuran
(8.89 g), hexane (10.2 kg), and n-butyllithium (5.22 mmol, n-hexane
solution). The mixture was subjected to polymerization with stirring at
65° C. for three hours. After completion of the polymerization,
N,N-dimethylaminopropylacrylamide (1.57 mmol, 0.245 g) and
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate (3.66 mmol, 2.251 g) were
added thereto. After the reaction was allowed to proceed with stirring
for 30 minutes, methanol (10 ml) was added thereto and the mixture was
further stirred for five minutes. Then, the contents were taken out from
the polymerization reactor, and 2,6-di-t-butyl-p-cresol (10 g, SUMILIZER
BHT produced by Sumitomo Chemical CO., Ltd., the same shall apply
hereinafter) and oil (141 g) were added thereto. Most of hexane was
distilled out and the resulting mixture was dried under reduced pressure
at 55° C. for 12 hours to give a rubber mixture 1.

Preparation Example 2

[0087] A rubber mixture 2 was prepared in the same manner as in
Preparation Example 1, except that the amount of
N,N-dimethylaminopropylacrylamide was changed to 0.52 mmol (0.082 g) and
the amount of 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed
to 4.70 mmol (2.894 g).

Preparation Example 3

[0088] A rubber mixture 3 was prepared in the same manner as in
Preparation Example 1, except that the amount of
N,N-dimethylaminopropylacrylamide was changed to 4.70 mmol (0.734 g) and
the amount of 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed
to 0.52 mmol (0.322 g).

Preparation Example 4

[0089] A rubber mixture 4 was prepared in the same manner as in
Preparation Example 1, except that the amount of tetrahydrofuran was
changed to 31.12 g; the amount of n-butyllithium (n-hexane solution) was
changed to 18.28 mmol; the amount of N,N-dimethylaminopropylacrylamide
was changed to 1.83 mmol (0.286 g); and the amount of
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed to 16.45 mmol
(10.131 g).

Preparation Example 5

[0090] A rubber mixture 5 was prepared in the same manner as in
Preparation Example 1, except that the amount of tetrahydrofuran was
changed to 31.12 g; the amount of n-butyllithium (n-hexane solution) was
changed to 18.28 mmol; the amount of N,N-dimethylaminopropylacrylamide
was changed to 16.45 mmol (2.57 g); and the amount of
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed to 1.83 mmol
(1.126 g).

Preparation Example 6

[0091] A rubber mixture 6 was prepared in the same manner as in
Preparation Example 1, except that the amount of tetrahydrofuran was
changed to 4.15 g; the amount of n-butyllithium (n-hexane solution) was
changed to 2.44 mmol; the amount of N,N-dimethylaminopropylacrylamide was
changed to 0.24 mmol (0.038 g); and the amount of
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed to 2.19 mmol
(1.351 g).

Preparation Example 7

[0092] A rubber mixture 7 was prepared in the same manner as in
Preparation Example 1, except that the amount of tetrahydrofuran was
changed to 4.15 g; the amount of n-butyllithium (n-hexane solution) was
changed to 2.44 mmol, the amount of N,N-dimethylaminopropylacrylamide was
changed to 2.19 mmol (0.343 g); and the amount of
1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed to 0.24 mmol
(0.15 g).

Preparation Example 8

[0093] A rubber mixture 8 was prepared in the same manner as in
Preparation Example 1, except that the amount of
N,N-dimethylaminopropylacrylamide was changed to 0 mmol (0 g), and the
amount of 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed to
2.44 mmol (1.501 g).

Preparation Example 9

[0094] A rubber mixture 9 was prepared in the same manner as in
Preparation Example 1, except that the amount of
N,N-dimethylaminopropylacrylamide was changed to 2.44 mmol (0.381 g), and
the amount of 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate was changed
to 0 mmol (0 g).

Preparation Example 10

[0095] A 20-L stainless steel polymerization reactor was cleaned and
dried, and the air therein was replaced with dry nitrogen. To the reactor
were then added 1,3-butadiene (548.3 g), styrene (235 g), tetrahydrofuran
(8.89 g), hexane (10.2 kg), and n-butyllithium (5.22 mmol, n-hexane
solution). The mixture was subjected to polymerization with stirring at
65° C. for three hours. After completion of the polymerization,
N,N-dimethylaminopropylacrylamide (5.22 mmol, 0.816 g) was added thereto.
After the reaction was allowed to proceed with stirring for 30 minutes,
methanol (10 ml) was added thereto and the mixture was further stirred
for five minutes (modified diene rubber A).

[0096] Separately, a 20-L stainless steel polymerization reactor was
cleaned and dried, and the air therein was replaced with dry nitrogen. To
the reactor were then added 1,3-butadiene (548.3 g), styrene (235 g),
tetrahydrofuran (8.89 g), hexane (10.2 kg), and n-butyllithium (5.22
mmol, n-hexane solution). The mixture was subjected to polymerization
with stirring at 65° C. for three hours. After completion of the
polymerization, 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate (5.22
mmol, 3.216 g) was added thereto. After the reaction was allowed to
proceed with stirring for 30 minutes, methanol (10 ml) was added thereto
and the mixture was further stirred for five minutes (modified diene
rubber B).

[0097] Then, the contents were taken out from each polymerization reactor
and these two mixtures were mixed together so that the compounding ratio
of the modified diene rubbers A and B (weight ratio of A/B) was 30/70.
Thereto were added 2,6-di-t-butyl-p-cresol (10 g) and oil (141 g). Most
of hexane was distilled out and the resulting mixture was dried under
reduced pressure at 55° C. for 12 hours to give a rubber mixture
10.

Examples and Comparative Examples

[0098] The chemical agents in formulation amounts shown in Table 1 or 2,
except the sulfur and vulcanization accelerator, were kneaded with a
Banbury mixer at 150° C. for five minutes to give a kneaded
mixture. To the kneaded mixture were then added the sulfur and
vulcanization accelerator and the resulting mixture was kneaded with an
open two roll mill at 50° C. for five minutes to provide an
unvulcanized rubber composition. A portion of the obtained unvulcanized
rubber composition was press-vulcanized at 170° C. for 15 minutes
to provide a vulcanized rubber composition.

[0099] Another portion of the unvulcanized rubber composition was
processed into a tread shape, assembled with other tire components, and
vulcanized at 170° C. for 15 minutes to provide a test tire (tire
size: 195/65R15).

[0100] The obtained vulcanized rubber compositions and test tires were
evaluated as follows. The results are shown in Tables 1 and 2.

(Grip Index)

[0101] The test tire was mounted on each wheel of a vehicle (FF, 2000 cc,
made in Japan), and the vehicle was driven on an asphalt test course (dry
road surface). A test driver evaluated the stability of steering control
during the driving. The result of each tire was expressed as an index
value relative to a value of 100 representing the stability of
Comparative Example 1. A larger index value corresponds to a higher level
of grip performance (handling stability) on a dry road surface.

(Abrasion Index)

[0102] The Lambourn abrasion loss of each vulcanized rubber composition
was measured with a Lambourn abrasion tester under the conditions of room
temperature, an applied load of 1.0 kgf, and a slip ratio of 30%. Then, a
volume loss was calculated from the measured Lambourn abrasion loss. The
volume loss of each formulation was expressed as an index value relative
to a value of 100 representing the volume loss of Comparative Example 1
by the following equation. A larger index value means better abrasion
resistance.

[0103] Using a viscoelastic spectrometer VES (produced by Iwamoto
Seisakusho), the tan δ of each vulcanized rubber composition was
measured under the conditions of a temperature at 70° C., an
initial strain of 10%, and a dynamic strain of 2%. The measured value was
expressed as an index value relative to a value of 100 representing the
tan δ of Comparative Example 1 by the following equation. A larger
index value corresponds to a higher level of performance in terms of
rolling resistance (lower rolling resistance).

[0104] Tables 1 and 2 show that the grip performance, the abrasion
resistance, and the performance in terms of rolling resistance were
improved in a balanced manner in the Examples, while these performance
properties were not achieved in a balanced manner in the Comparative
Examples. From the results of Comparative Example 1 where neither of the
rubbers A and B was used, Comparative Example 6 where only the rubber B
was used, Comparative Example 7 where only the rubber A was used, and
Example 1 where both of the rubbers A and B were used, it was
demonstrated that the use of both rubbers synergistically improves the
balance among the performance properties, especially in terms of abrasion
resistance and rolling resistance.

[0105] From the results of comparing Examples 4 and 9 or Examples 3 and
10, it was also demonstrated that the addition of an aromatic vinyl
polymer improves the grip performance while maintaining performance in
terms of abrasion resistance and rolling resistance, thereby improving
the trade-off between these properties.

Patent applications by Michio Hirayama, Kobe-Shi JP

Patent applications in class At least one of these polymers is derived from two or more reactants

Patent applications in all subclasses At least one of these polymers is derived from two or more reactants